1. Technical Field
The invention relates to a piezoelectric actuator, a robot hand, and a robot.
2. Related Art
A piezoelectric actuator which uses resonance of a vibrating body including a piezoelectric device is known (for example, see JP-A-2000-092869). In such a piezoelectric actuator, a control unit performs control so that the frequency of a driving signal supplied to a vibrating body approaches a resonance frequency, and a phase difference between the driving signal and a detection signal obtained from a vibrating state of the vibrating body becomes substantially constant at a value appropriate for driving. In this way, a stable driving state where a driven body rotates stably is maintained.
The piezoelectric actuator (ultrasonic motor) disclosed in JP-A-2000-092869 includes a switching means for switching a driving circuit and a temperature sensor (thermistor) detecting the temperature of a transformer. When the temperature of the switching means or the transformer increased to a certain temperature or higher, the temperature rise is prevented by stopping or decreasing the output to thereby improve reliability.
However, the resonance frequency of the vibrating body varies with a change of a surrounding temperature or a load, and the resonance frequency decreases as the temperature increases. When the resonance frequency decreases, the amplitude of the vibrating body decreases, and the rotating speed of the driven body decreases. Thus, when the power of the driving signal is increased in order to maintain the rotating speed, the vibrating body generates heat so that the temperature increases further, and the resonance frequency decreases further. This occurs repeatedly so that the system goes into a negative control chain. As a result, there are cases were the vibrating body may be damaged.
In contrast, in the piezoelectric actuator disclosed in JP-A-2000-092869, since the piezoelectric actuator does not include a temperature sensor detecting the temperature of the vibrating body, it is not possible to detect a temperature rise in the vibrating body. Moreover, even when the piezoelectric actuator includes a temperature sensor detecting the temperature of the vibrating body, the circuit configuration of the control unit becomes complicated as compared to when the temperature sensor is not included. Thus, it is difficult to decrease the size and weight of the piezoelectric actuator, and the cost increases.